Cryptosporidiosis, caused by the protozoan parasite, Cryptosporidium parvum, is self-limited in normal hosts but can cause life threatening, chronic diarrhea in AIDS patients. No safe and effective treatment has been successfully developed for cryptospofidiosis associated with advanced AIDS. C. parvum infection causes intestinal physiologic changes like, increased chloride anion secretion (CI) and epithelial barrier disruption that leads to watery diarrhea. Substance P (SP), a neuropeptide, is a pain transmitter and can cause C1- ion secretion in human intestinal explants. We have previously studied SP expression in jejunal biopsies of AIDS patients with natural severe cryptosporidiosis and normal volunteers experimentally challenged with C. parvum (mild disease). SP expression was stronger in AIDS patients compared to normal volunteers with mild self-limited cryptosporidiosis. We hypothesize that SP is a key mediator of chronic intestinal symptoms in AIDS associated cryptosporidiosis. We also hypothesize that SP expression will be elevated in intestinal tissues of immunodeficient hosts because of cryptosporidiosis infection, HIV infection alone will not cause increased SP expression. To verify these hypotheses, we propose to use an immunodeficient animal model of cryptosporidiosis, ie. primates with AIDS (after experimental SIV infection) and cryptosporidiosis as an opportunistic naturally occurring infection. Advantage of an animal model is that, it is easier to procure large tissue samples from an animal model to that from AIDS patients with cryptosporidiosis, and, studies aimed at defining molecular targets responsible for disease pathogenesis and initial therapeutic testing of specific antagonists can best be studied using animal derived tissues. The goal of this project is to test the hypothesis that SP mediates severe symptoms of cryptosporidiosis in immunodeficient hosts. Specific aim 1: To determine if intestinal SP is upregulated in immunodeficient animals with chronic naturally infected cryptosporidiosis as compared to immunodeficient animals without cryptosporidiosis or normal immunocompetent macaques with subclinical experimental cryptosporidiosis. Ileal expression of SP mRNA and protein levels will be compared between immunodeficient macaques (with AIDS) with and without naturally occurring C. parvum infection and in normal macaques with and without subclinical experimental C. parvum infection. Specific aim 2: To test the hypothesis that SP is a key factor that mediates intestinal physiological alterations that lead to watery diarrhea in naturally occurring chronic cryptosporidiosis associated with immunodeficient hosts. C1- ion secretion and barrier integrity will be compared between ileal tissues from SIV infected macaques (with AIDS) with and without naturally occurring C. parvum infection in the presence and absence of SP receptor antagonist by the Ussing chamber technique. These studies will determine the role of SP in the pathogenesis of C. parvum induced diarrhea. Evidence implicating SP in the disease process would support the use of SP receptor antagonists as a therapy for the life threatening illness associated with AIDS related cryptosporidiosis and perhaps other intestinal pathogens.